In order to cathodically protect steel it is necessary to place it in a negative potential, at least of -800 mv, said protential having to refer to a reference silver-silver chloride electrode.
Cathodic protection can be carried out according to two well defined techniques, one called sacrifice anode and the other by printed current.
When cathodic protection is carried out with a sacrifice anode, the same is based on connecting the steel to be protected to some metals more negative than it, in such a way that with an adequate amount of said metals the desired potential of -800 mv will be achieved.
For said purpose three types of anodes can be used: magnesium anodes (-1,600 mv), aluminum anodes (-1,150 mv) and zinc anodes (-1,050 mv.) These anodes wear down or are sacrificed to protect steel which in this case is cathode, hence the name of sacrifice anode.
Aluminum anodes have in the last few years been used more and more due to their long life and high electro-chemical capacity, although whether they are made out of this material or whether they are made out of the previously cited materials, sacrifice anodes normally have two inconveniences which should be taken into account:
Current output: upon the difference of potential between a sacrifice anode and the steel to be protected fixed, the current output of a specific anode will depend on the electrical resistance of the circuit and thus mainly on the resistivity of the electrolyte (sea water or other water which the ship sails in.) Therefore, a ship that sails in water with a variable resistivity (sea, rivers, etc.) the sacrifice anodes will give a variable current, whereby the cathodic protection will only be effective in waters for which they were designed. The ship may remain without cathodic protection due to the lack of current or on the contrary an excessive production of current with very exaggerated wear of the anodes and a shorter duration thereof.
Anode duration: they wear down and thus a useful life that tends to be the time that passes from the installment up to 85% consumption (utilization factor) in which the anode wears down and has to be replaced by another one, which requires biannual, annual replacement, etc., depending on the size of the anode itself.
When cathodic protection is done by means of printed current, the system is based on applying a negative potential to the steel structure to be protected (cathod) until placing it at a potential of -800 mv, thus an outside electric source is needed and once it is corrected it will be applied in such a way that the negative pole is connected to the steel and the positive pole to the electrolyte (water) by means of some inert electrodes without hardly any wear which, in this case, will be called inert anodes. These anodes have been used being made out of corrosion resistant materials, such as titanium coated with a layer of ruthenium.
The printed current has had an advancement that has been marked by the development of automatic or manual rectifiers, reference electrodes to control potential and above all the important technological development of the latest inert anodes comprised of titanium, development due to electrolysis processes within the large worldwide chemical undertaking. In this sense W.W.I. developed years ago the so-called AQUAMATIC" equipment based on high power platinum-plated titanium anodes (50 or more amperes per anode), automatic rectifiers and zinc reference electrodes that keep the control system of the rectifier informed at all times of the potential of the ship to be protected and gives the necessary orders so that with or less current coming out of the anodes the electric potential of the anode is kept between -800 and -1.500 volts. This equipment that operates correctly in thousands of large ships has the property of automatically adjusting itself to the different water which the ship sails in, even at a variable speed of the ship, in order to keep the potential of the underwater hull of said ship within the established limits. Hence, such equipment is costly or needs maintenance by technicians and a permanent electric energy source.
On the other hand, it must be taken into account that in present-day systems, the potential of the cathode (hull of the ship) is kept between -800 and -1.500 mv, a thing that is obtained by means of a reference electrode that reads the potentials and gives the orders to the automatic regulator so that more or less current is drawn and thus the potentials are kept within the programmed limits. If such an automatism does not exist, they would either remain below -800 mv., whereby the hull would rust, or they could exceed -2,000 mv., whereby the hull would be overprotected and the paint of the ship could peel due to the liberation of hydrogen in the cathode.
It also has to be kept in mind that in conventional cathodic protection systems, the anode-cable connection is done by means of a steel box, said box being welded to the hull of the ship in the inside, while the titanium anode sticks out into the sea through a rather large hole, in such a way that said system involves control and maintenance from inside the ship. The operating responsibility of the system in question, has a second steel seal inside the hull, since if this were not the case there could be the risk that failure of the system could cause sea water to enter with the subsequent danger of buoyancy of the ship, thus this system is costly and requires periodic checks and maintenance.